Dose management during the Borssele modifications29 October 1998
The large-scale backfitting operation carried out at the Borssele nuclear power plant in 1997, a major part of the plant’s Modification Project, took approximately four months. Managing dose was a critical consideration in planning and executing the work.
It was clear right from the start of planning the Modification Project that relatively high collective and individual doses would be received during the work and that it was of the utmost importance to monitor these doses carefully to be able to react as soon as possible in case of deviations. This dictated that we take a different approach than normally applied at the plant for radiological protection. Detailed planning of the expected dose uptake, daily monitoring of the dose and consulting with the operational staff were undertaken to ensure that the dose received remained within the estimated dose limits.
RADIATION PROTECTION PREPARATION
In 1993 a first dose estimate was made which yielded 10 person-Sv. At Borssele the ALARA principle is applied to keep the doses as low as is reasonably achievable. The monetary value for the Sievert, ie the value in Dutch guilders which may be spent on measures to limit the dose, is between NLG 1000 and 2000 ($540-1080) per mSv saved. The main contractor for the job, Siemens/KWU, was committed to plan dose limiting measures in accordance with the ALARA directives applicable at Borssele, and to apply the monetary value per Sievert.
One planned project – the relocation of the branching of the emergency core cooling system in the primary system – was cancelled on the basis of the ALARA justification principle. The safety gain (reduction of the melting frequency) calculated using the probabilistic safety assessment (PSA) specific for the Borssele station, was deemed insufficient to justify the collective dose necessary for the relocation of this branching.
After ALARA reports were drawn up for all eligible sub-projects and the procedures were determined, the dose estimate was 5 person-Sv.
According to the ALARA procedure applied (see previous article) a first ALARA report is used to select the best from several technical solutions, partly on the basis of ALARA considerations (ALARA part 1). A second ALARA report was then drawn up for the solution selected in order to optimise the procedure, with regard to the dose, even further through a closer examination of procedures, elapsed time, shielding concept and possible decontamination (ALARA part 2).
After several optimisation rounds on the basis of the above-mentioned ALARA reports, the dose estimate for the part of the modifications which were executed in 1997, was 1.85 person-Sv.
Together with the 0.725 person-Sv received in the period from 1994 to 1996 this led to an expected collective dose of 2.575 person-Sv for the entire project.
The most important modifications in respect of radiation load for the 1997 work are listed in the top panel, along with the dose management tools applied.
An example of how radiation protection was applied is the primary safety valves modification sub-project (YP) is described in the lower panel.
RESULTS AND LESSONS
As a result of the detailed dose planning of the work involved, taking account of variations over time, and the daily monitoring and subsequent discussions over dose, estimated and actual, with the departments executing the work, there was at all times a good overview of the dose situation. Any deviations detected would be discussed with the relevant departments immediately and corrective measures could be taken.
This way of working has contributed significantly to ensuring that the total dose actually received in connection with the Modifications Project remained within the estimated dose.
A positive side-effect of the intensive consultations described above, is that a good relationship developed between the Radiation Protection Department and the departments doing the work. The result was that these departments started to regard dose reduction as an essential part of their daily task even more than before.
Estimating and monitoring the dose in such an intensive way is only useful if it concerns a relatively long-term project from which a significant dose is expected, as the preparations required and the monitoring of the project involve a considerable amount of work. In the future other eligible projects will be approached in the same way.
|The primary safety valves modification sub-project (YP)|
|The YP sub-project comprised the removal of the old piping and the safety valves and the fitting of the new installation. For the whole sub-project a total of 1790 man hours were estimated. For the removal, 444 hours were projected, and for the fitting of the new piping and valves, 1000 hours. The remaining time was to deal with so-called ‘obstacles’ (eg the re-routing of pipes from another system which were blocking the way). In order to limit the dose received as much as possible the first operation was the removal of the radiating pipes and valves. These components were sawed out of the system and instantly carried off as waste. The average dose rate in the work space during the removal of the old pipes and valves was approx 0.40 mSv/h. During the fitting of the new pipes and valves the average dose rate in the work space was approx 0.25 mSv/h with some local peaks of 1 mSv/h. These dose rates applied even though an optimum lead shielding (approx 23 t) was fitted. The initial estimate of the dose for the modification described above was 0.54 person-Sv. The anticipated dose variations over time corresponded to that shown in the graph. During the implementation of the sub-project the daily dose sustained was compared to the dose estimated for that day. Possible changes in the planning were immediately taken account of in the estimate. On account of the extremely detailed dose planning and the detailed monitoring of the dose received, deviations of the actual dose compared to the estimate were identified immediately. When deviations were detected, these were immediately discussed with the executive departments, after which the necessary corrective steps were taken. Right at the start of the work it appeared that the removal of the old piping cost more dose than had been initially estimated. As soon as this was discovered, the Radiation Protection Department consulted with the contractor to find the cause and consider a change in the procedure. In this case the procedure appeared to be optimised sufficiently, however, and the dose estimate was adapted on the basis of experience acquired. The new dose estimate was 0.595 Sv; the actual dose was 0.586 Sv. During the entire project daily consultations were conducted with the contractors on the basis of the dose estimated and the dose received.|